Disk drives are commonly used to store large amounts of data in a readily available form. Typically, the primary components of a disk drive are a head disk assembly and a printed circuit board assembly which, when fixed to one another, form a functional unit that is then connected to a computer. The head disk assembly includes a head, and at least one data storage disk mounted on a spindle motor for rotating the storage disk near the head so that read/write operations may be performed on the disk.
Conventionally, disk surfaces are partitioned into logical zones for optimizing storage capacity by varying bit density within each of the logical zones. The zones may be visualized as concentric bands of tracks with a varying progression of bit density from band to band. In addition, disk surfaces are further partitioned via a series of angularly-spaced embedded servo-wedges disposed on the disk surfaces between data-wedges which contain data tracks with data sectors recorded in the intervals between servo-wedges on each track. Typically, servo-wedges are sampled at regular intervals by a read/write channel, and are processed by a servo controller to provide position information to a microprocessor for positioning a head over a desired track.
In order to reliably process the servo-wedge information in a given zone, the head gain associated with the zone is calibrated during disk drive manufacturing and stored in non-volatile memory such as reserved cylinders on the disk. Conventionally, the disk drive control system will retrieve the calibrated head gain along with other calibrated settings and the head gain for each zone is checked against the calibrated values during each initialization of the disk drive. An Automatic Gain Control (AGC) field is provided in each servo wedge which is read by the head and based on which the control system adjusts the head gain prior to reading data. In order to expedite the head gain adjustments at each servo-wedge, the pre-stored initial servo channel gain value is often used to reduce the time required for an auto ranging amplifier in the control system read channel to settle on an optimized gain.
Unfortunately, checking the head gain calibration for each zone and each head in a disk drive delays the time required for disk drive initialization. A typical disk drive currently may have 20 or more logical zones on each surface, while system manufacturers are demanding shortened initialization periods for the disk drive to provide a faster boot-up for the user. It is necessary to check the heads because over time, a phenomenon known as “head degradation” may occur which causes the head to operate at a significantly changed gain level.
Accordingly, what is needed is a method to reduce the time associated with the head gain calibration while monitoring for degradation of the head values over time.